In rolling-element bearings the surfaces of the rings (or of the raceways) as well as of the rolling elements are often coated in order to achieve a protection of the surfaces or an improved function of the rolling-element bearing.
For example, the surfaces, in particular of the rolling elements, can be subjected to a black oxide process such as a bluing in order to produce a multifunctional layer that protects the rolling elements. This frequently occurs in connection with fully blued bearings. Bluing offers a reduction of damage mechanisms, for example, a corrosion protection of the surfaces. In addition, bluing improves the run-in properties of the surfaces.
Furthermore, there is the possibility of providing the rolling elements as well as the bearing rings with a phosphate coating. A sliding or separating layer is generated by such a phosphate coating between the mutually rubbing surfaces of the rolling-element bearing, i.e., between the rolling elements and the raceways.
Both with a bluing and with a phosphate coating the raceways of the bearing rings and the counter-races of the rolling elements are each coated with the same system, provided all surfaces are coated. Here the bluing is generated in an alkaline manner and is thus soluble in acid, whereas a phosphate coating is generated acidically and is thus alkali-soluble. A bearing including blued or phosphated surfaces is therefore either only alkaline or acid-resistant. If the rolling-element bearing is exposed to a corresponding acidic or alkaline environment, the coatings of the surfaces are therefore impaired or eliminated, and the surfaces of the bearing rings and of the rolling elements can therefore no longer be held separate from each other depending on the chemical action, i.e., they rub directly against each other or have metallic contact. The service life of the bearing or of the individual components can thus be reduced.